Zro2-al2o3 composite ceramic material and production method therefor

ABSTRACT

A ZrO 2 —Al 2 O 3  composite ceramic material having excellent wear resistance, hardness, strength and toughness is provided. This ceramic material comprises a ZrO 2  phase composed of 90 vol % or more of tetragonal ZrO 2 , and containing 10 to 12 mol % of CeO 2  as a stabilizer, and an Al 2 O 3  phase. An amount of the Al 2 O 3  phase in the ceramic material is in a range of 20 to 70 vol %, and preferably 40 to 70 vol %. In the composite ceramic material, Al 2 O 3  grains each having a fine ZrO 2  grain therein are dispersed. Some of the Al 2 O 3  grains each having the fine ZrO 2  grain therein are trapped within ZrO 2  grains to form composite grains. A ratio of the number of the Al 2 O 3  grains each having the fine ZrO 2  grain therein relative to the number of the entire Al 2 O 3  grains dispersed in the composite ceramic material is 10% or more, and preferably 50% or more.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation patent application of U.S. patent applicationSer. No. 12/123,356 filed on May 19, 2008, now allowed, which was acontinuation-in-part application of U.S. patent application Ser. No.11/085,039 filed on Mar. 22, 2005, now abandoned, which claims thepriority of Japanese Patent Application No. 2004-085102 filed on Mar.23, 2004, the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ZrO₂—Al₂O₃ composite ceramic materialwith excellent mechanical properties, and a method of producing thesame.

2. Disclosure of the Prior Art

As compared with metal and plastic materials, ceramic materialsdemonstrate excellent hardness, wear resistance, heat resistance andcorrosion resistance. As for practical use of the ceramic materials invarious application fields, for example, biomaterial parts such asartificial joint and artificial tooth, medical equipments, cutting toolssuch as drill and surgical knife, mechanical parts for automobile,airplane and space craft used under severe conditions, it is desired todevelop a ceramic material having further improved mechanical strengthand toughness in a high level. In recent years, a zirconia(ZrO₂)-alumina (Al₂O₃) composite ceramic material has received attentionas a potential candidate of the ceramic material.

For example, Japanese patent Early publication [kokai] No. 5-246760discloses a ZrO₂-based composite ceramic sintered body comprising amatrix phase of tetragonal ZrO₂ grains containing 5 to 30 mol % of CeO₂and a dispersion phase of fine grains of at least one of selected fromthe group consisting of Al₂O₃, SiC, Si₃N₄ and B₄C, which are dispersedwithin the ZrO₂ grains and at grain boundaries of the matrix phase. Bythe presence of the dispersion phase, a grain growth of the matrix phaseis prevented and a fine grained structure of the matrix phase isobtained, so that significant strengthening is achieved mainly due to areduction in flaw size of the fracture origin.

In addition, U.S. Pat. No. 5,728,636 discloses a ZrO₂-based ceramicmaterial having high mechanical strength and toughness, which comprisesa tetragonal ZrO₂ phase of ZrO₂ grains having an average grain size of 5μm or less, and containing 8 to 12 mol % of CeO₂ and 0.05 to 4 mol % ofTiO₂ as a stabilizer, and an Al₂O₃ phase of Al₂O₃ grains having anaverage grain size of 2 μm or less. In this ceramic material, the Al₂O₃grains are dispersed within the ZrO₂ grains at a dispersion ratio of 2%or more, which is defined as a ratio of the number of the Al₂O₃ grainsdispersed in the ZrO₂ grains relative to the number of the entire Al₂O₃grains dispersed in the ceramic material. In addition, by using thecombination of CeO₂ and TiO₂ as the stabilizer, the grain growth of theZrO₂ grains can be enhanced moderately, so that parts of the Al₂O₃grains are effectively dispersed within the ZrO₂ grains, and a criticalstress of a stress induced transformation from the tetragonal ZrO₂ tomonoclinic ZrO₂ is increased.

By the way, as one potential approach for improving wear resistance andhardness of this kind of the ceramic material, it is proposed toincrease the additive amount of Al₂O₃. However, such an increase of theAl₂O₃ amount generally leads to reductions in mechanical strength andtoughness. In above cases, a preferred amount of Al₂O₃ in the compositeceramic sintered body or the ceramic material is in a range of 0.5 to 50vol %. When the Al₂O₃ amount exceeds 50 vol %, Al₂O₃ becomes the matrixphase, so that it is difficult to maintain a strengthening mechanismbased on a stress induced phase transformation of ZrO₂. Consequently,considerable reductions in mechanical strength and toughness may occur.

Thus, the previous ZrO₂—Al₂O₃ composite ceramic materials still have aproblem to be solved for providing excellent wear resistance andhardness without causing reductions in mechanical strength and toughnessunder a larger amount of Al₂O₃.

SUMMARY OF THE INVENTION

Therefore, a primary concern of the present invention is to provide aZrO₂—Al₂O₃ composite ceramic material having increased wear resistanceand hardness, while maintaining a good balance between mechanicalstrength and toughness, under a larger amount of Al₂O₃ than heretofore.

That is, the ZrO₂—Al₂O₃ composite ceramic material of the presentinvention comprises a ZrO₂ phase containing 10 to 12 mol % of CeO₂ as astabilizer and composed of 90 vol % or more of tetragonal ZrO₂, and anAl₂O₃ phase. An amount of the Al₂O₃ phase in the composite ceramicmaterial is in a range of 20 to 70 vol %, and preferably 40 to 70 vol %.Al₂O₃ grains each having a fine ZrO₂ grain therein are dispersed in thecomposite ceramic material. Some of the Al₂O₃ grains each having thefine ZrO₂ grain therein are trapped within ZrO₂ grains to form compositegrains. A first dispersion ratio defined as a ratio of the number of theAl₂O₃ grains each having the fine ZrO₂ grain therein relative to thenumber of the entire Al₂O₃ grains dispersed in the composite ceramicmaterial is 10% or more, preferably 30% or more, and more preferably 50%or more. A second dispersion ratio defined as a ratio of the number ofthe Al₂O₃ grains of the composite grains relative to the number of theentire Al₂O₃ grains dispersed in the composite ceramic material is 0.3%or more, preferably 1% or more, and more preferably 1.5% or more.

According to the present invention, the Al₂O₃ grains each having thefine ZrO₂ grain therein are dispersed at 10% or more of the firstdispersion ratio in the ZrO₂—Al₂O₃ composite ceramic material. This is,by using a specific production method described later, an amount ofso-called zirconia toughened alumina (ZTA) grains, which are formed bytrapping fine tetragonal ZrO₂ grains within Al₂O₃ grains, can beremarkably increased. This brings about an increase in critical stressrequired for causing stress-induced phase transformation from thetetragonal ZrO₂ to the monoclinic ZrO₂. As a result, it is possible toreliably provide the composite ceramic material with excellentmechanical strength and toughness.

Without wishing to be bound by theory, it is presently believed that theremarkable improvement of mechanical properties in the present inventionresults from the following mechanism. As described above, the ZrO₂—Al₂O₃composite ceramic material of the present invention is characterized inthat the Al₂O₃ grains having the fine (tetragonal) ZrO₂ grains thereinare dispersed at a high dispersion ration (first dispersion ratio) inthe ZrO₂—Al₂O₃ composite ceramic material. Since the Al₂O₃ grains havingthe fine (tetragonal) ZrO₂ grains therein construct the zirconiatoughened alumina (ZTA) grains, the toughness of the Al₂O₃ grain isremarkably improved by the presence of the fine ZrO₂ grain. Moreover,when such a toughness-improved Al₂O₃ grain is trapped within the(larger-sized) ZrO₂ grain, sub-grain boundaries are formed within theZrO₂ grain. The formation of the sub-grain boundaries plays a role individing the (larger) ZrO₂ grain incorporating the toughness-improvedAl₂O₃ grain therein into imaginary more finer sized grains.

Thus, by achieving a remarkably high dispersion ratio of the ZTA grains,i.e., 10% or more of the first dispersion ratio, it is possible toprovide the ZrO₂—Al₂O₃ composite ceramic material having excellent wearresistance and hardness, while maintaining the good balance betweenmechanical strength and toughness, even under a larger amount of 40 to70 vol % of the Al₂O₃ phase in the composite ceramic material.

In the above composite ceramic material, when ZrO₂ grains each having anAl₂O₃ grain therein are dispersed in the composite ceramic material, anda third dispersion ratio defined as a ratio of the number of the Al₂O₃grains trapped within the ZrO₂ grains relative to the number of theentire Al₂O₃ grains dispersed in the composite ceramic material is 1.5%or more, it is preferred that the second dispersion ratio “R2” satisfiesthe following condition:

R2≧α×R3×R1/100

wherein “R1” is the first dispersion ratio, “R3” is the third dispersionratio, and “α” is a coefficient determined within a range of 0.8 to 1 bytaking account of measurement errors.

A further concern of the present invention is to provide a method ofproducing the ZrO₂—Al₂O₃ composite ceramic material described above.That is, this method comprises the steps of:

mixing a first powder for providing the ZrO₂ phase with a second powderfor providing the Al₂O₃ phase such that an amount of the Al₂O₃ phase inthe composite ceramic material is in a range of 20 to 70 vol %;molding a resultant mixture in a desired shape to obtain 3 greencompact; and sintering said green compact in an oxygen-containingatmosphere,

wherein the second powder has Al₂O₃ particles each having a fine ZrO₂particle therein, and

a ratio of the number of the Al₂CO₃ particles each having the fine ZrO₂particle therein in the second powder is 5% or more.

According to the production method of the present invention, a highdispersion ratio of the ZTA grains, i.e., 10% or more of the firstdispersion ratio can be achieved by using the second powder with 5% ormore of the Al₂O₃, particles each having a fine ZrO₂ particle therein,and trapping fine ZrO₂ grains within Al₂O₃ grains by help of graingrowth during the sintering process.

As a preferred preparation process of the second powder in the abovemethod, it comprises the step of adding a ZrO₂ powder to at least oneselected from a θ-Al₂O₃ powder and a γ-Al₂O₂ powder having, a specificsurface of 50 to 400 m²/g to obtain a mixed powder. In addition, it ispreferred that the preparation process comprises the steps of adding aZrO₂ powder to one of an aqueous solution of an aluminum salt and anorganic solution of an aluminum alkoxide, hydrolyzing a resultantmixture to obtain a precipitate, and drying the precipitate.Alternatively, it is preferred that the preparation process comprisesthe steps of adding an aqueous solution of a zirconium salt to one of anaqueous solution of an aluminum salt and an organic solution of analuminum alkoxide, hydrolyzing a resultant mixture to obtain aprecipitate, and drying the precipitate. In theses preparationprocesses, it is preferred to calcine the mixed powder or theprecipitate in an oxygen containing atmosphere it a temperature of 800°C. to 1300° C.

These and further purposes and advantages of the present invention willbe clearly understood from the following detail explanation and examplesof the invention.

BRIEF EXPLANATION OF THE DRAWING

The FIGURE is a SEM photograph showing a composite grain dispersed in aZrO₂—Al₂O₃ composite ceramic material of the present invention.

DETAIL EXPLANATION OF THE INVENTION

The ZrO₂ phase of the ZrO₂—Al₂O₃ composite ceramic material of thepresent invention is composed of 90 vol % or more of tetragonal ZrO₂. Toobtain such a large amount of tetragonal ZrO₂, the ZrO₂ phase contains10 to 12 mol % of CeO₂ as a stabilizer. When the CeO₂ amount is lessthan 10 mol %, an amount of monoclinic ZrO₂ relatively increases, sothat cracks may easily occurs in the composite ceramic material. On theother hand, when the CeO₂ amount exceeds 12 mol %, cubic ZrO₂ of ahigh-temperature stable phase begins to appear, so that there is a fearthat the mechanical strength and toughness can not be sufficientlyimproved by the stress induced phase transformation of tetragonal ZrO₂to monoclinic ZrO₂. Preferably, the zirconia phase is composed of 90 vol% or more of tetragonal ZrO₂ and the balance of monoclinic ZrO₂.

The composite ceramic material of the present invention is essential tocontain 20 to 70 vol %, preferably 40 to 70 vol %, and more preferablymore than 50% and less than 70% of the Al₂O₃ phase. When the Al₂O₃amount is less than 20 vol %, the wear resistance and the mechanicalstrength of the composite ceramic material can not be sufficientlyimproved. On the other hand, when the Al₂O₃ amount exceeds 70 vol %,considerable reductions in mechanical strength and toughness occur. Whenthe Al₂O₃ amount is in the range of 40 to 70 vol %, it is possible toprovide a high-reliability ceramic material having good balance betweenthe mechanical strength and toughness in a high level.

One of the important features of the composite ceramic material of thepresent invention is that Al₂O₃ grains each having a fine ZrO₂ graintherein are uniformly dispersed in the composite ceramic material at amuch higher dispersion ratio than heretofore by using an Al₂O₃ powdercontaining a composite powder in a production method described below forthe composite ceramic material of the present invention. In this regard,the composite powder is composed of Al₂O₃ particles each having a fineZrO₂ particle therein. An amount used of the composite powder isdetermined such that a ratio of the number of the Al₂O₃ particles eachhaving the fine ZrO₂ particle therein in the Al₂O₃ powder is 5% or more,preferably 20% or more, and more preferably 35% or more. In addition, aratio of the number of the Al₂O₃ grains each having, the fine ZrO₂ graintherein relative to the number of the entire Al₂O₃ grains dispersed inthe composite ceramic material is defined as a first dispersion ratio.In the present invention, the first dispersion ratio is 10% or more,preferably 30% or more, and more preferably 50% or more. A mechanism ofimproving mechanical properties of the composite ceramic materialresulting from an increase in the first dispersion ratio of the Al₂O₃grains each having the fine ZrO₂ grain therein in the composite ceramicmaterial is explained later in detail.

The composite ceramic material of the present invention is alsocharacterized in that composite grains, each of which has a structurethat an Al₂O₃ grain having a fine (tetragonal) ZrO₂ grain therein istrapped within a (larger-sized) ZrO₂ grain, as shown in the FIGURE, areaggressively dispersed in the composite ceramic material. In the presentspecification, a ratio of the number of the Al₂O₃ grains of thecomposite grains relative to the number of the entire Al₂O₃ grainsdispersed in the composite ceramic material is defined as a seconddispersion ratio. The second dispersion ratio is 0.3% or more. When thesecond dispersion ratio is less than 0.3%, the formation amount of thezirconia toughened alumina (ZTA) in the composite ceramic materialdecreases, so that there is a fear that the effect of improving themechanical strength and toughness is not sufficiently obtained asincreasing, Al₂O₃ content. In other words, as the second dispersionratio is 0.3% or more, preferably 1% or more, and more preferably 1.5%or more, higher improvements of both mechanical strength and toughnessof the composite ceramic material can be achieved.

In addition, when ZrO₂ grains each having, an Al₂O₃ grain therein aredispersed in the composite ceramic material, a ratio of the number ofAl₂O₃ grains trapped within ZrO₂ grains relative to the number of theentire Al₂O₃ grains dispersed in the composite ceramic material isdefined as a third dispersion ratio. In the present invention, it ispreferred that the third dispersion ratio is 1.5% or more. When thethird dispersion ratio is less than 1.5%, an effect of dividing the ZrO₂grains into more finer sized grains by the formation of sub-grainboundaries may become insufficient, so that a reduction in strengtheasily occurs as increasing Al₂O₃ content. An upper limit of the thirddispersion ratio is not restricted. In a theoretical sense, as the thirddispersion ratio increases, the mechanical properties of the compositeceramic material can be further improved. The number of Al₂O₃ grainseach existing in the composite grain is included in the number of theAl₂O₃ grains dispersed in the ZrO₂ grains.

In the present invention, it is preferred that the second dispersionratio “R2” satisfies the following condition:

R2≧α×R3×R1/100

wherein “R1” is the first dispersion ratio, “R3” is the third dispersionratio, and “α” is a coefficient determined within a range of 0.8 to 1 bytaking account of measurement errors. Thus, the composite ceramicmaterial of the present invention is further characterized in that theformation amount of the composite grains increases as the firstdispersion ratio and/or the third dispersion ratio becomes larger.

The size of the fine ZrO₂ grain of the composite grain is not restrictedon the assumption that the fine ZrO₂ grain can be trapped within theAl₂O₃ grain. For example, it is preferred that fine tetragonal ZrO₂grains having an average grain size of several ten nanometers aretrapped within the Al₂O₃ grains. The number of the fine ZrO₂ grains eachtrapped within the Al₂O₃ grain of the composite grain is included in thenumber of the ZrO₂ grains dispersed in the Al₂O₃ grains.

It is preferred that the Al₂O₃ grains of the composite ceramic materialhas an average grain size of 0.1 to 0.5 μm. When the average grain sizeexceeds 0.5 μm, it becomes difficult to disperse the Al₂O₃ grains withinthe ZrO₂ grains at the above third dispersion ratio. On the other hand,when the average grain size is less than 0.1 μm, it is difficult toobtain a highly dense sintered body of the composite ceramic material bypressureless sintering.

The size of the ZrO₂ grain of the composite grain is determined suchthat the Al₂O₃ grain having the fine ZrO₂ grain therein is trappedwithin the ZrO₂ grain. However, when the size of the ZrO₂ grain isexcessively large, it may lead to a reduction in strength of thecomposite ceramic material. From this viewpoint, it is preferred that anaverage grain size of the ZrO₂ grains of the composite ceramic materialis in a range of 0.1 to 1 μm. This average grain size is based on theZrO₂ grains other than the fine ZrO₂ grains trapped within the Al₂O₃grains. When the average grain size exceeds 1 μm, reductions in wearresistance and mechanical strength may occur. On the other hand, whenthe average grain size is less than 0.1 μm, it becomes difficult toobtain a highly dense sintered body of the composite ceramic material bypressureless sintering.

By the way, in the case of a conventional composite ceramic materialwith a simply mixed structure of ZrO₂ and Al₂O₃ grains having an averagegrain size of several micron levels, when the Al₂O₃ amount exceeds 30vol %, the toughening mechanism based on the stress induced phasetransformation of tetragonal ZrO₂ to monoclinic ZrO₂ is not a dominantfactor of the composite ceramic material, so that there is a tendencythat the mechanical strength and toughness gradually decrease. Inaddition, when the Al₂O₃ amount exceeds 50 vol %, it means that thematrix phase of the composite ceramic material is provided by the Al₂O₃phase. This leads to a considerable deterioration of the mechanicalproperties of the conventional composite ceramic material.

However, according to the ZrO₂—Al₂O₃ composite ceramic material of thepresent invention, the fine ZrO₂ grains each trapped within the Al₂O₃grain and the Al₂O₃ grains each trapped within the ZrO₂ grain contributeto promote piling up dislocations and form the sub-grain boundarieswithin the crystal grains, so that the wear resistance and mechanicalstrength of the composite ceramic material can be remarkably improved.In particular, when the Al₂O₃ amount is in the range of 40 to 70 vol %,fine tetragonal ZrO₂ grains are uniformly dispersed in the Al₂O₃ grainsto form the zirconia toughened alumina (ZTA) structure, so that theAl₂O₃ grains are remarkably reinforced. In other words, even when theAl₂O₃ amount exceeds 50 vol %, high mechanical strength and toughnesscan be maintained by the formation of a fine crystal-grain structureeffectively reinforced by the tetragonal ZrO₂ grains. From thesereasons, the ZrO₂—Al₂O₃ composite ceramic material of the presentinvention obtained under the Al₂O₃ content larger than 50 vol % wherethe matrix phase is the Al₂O₃ phase exhibits excellent mechanicalstrength and toughness substantially comparable to the former ZrO₂—Al₂O₃ceramic material where the matrix phase is the ZrO₂ phase.

Without wishing to be bound by theory, it is presently believed that themechanical properties of the composite ceramic material of the presentinvention are improved by the following mechanism. That is, when thecomposite grains are dispersed in the composite ceramic material, eachof which has the structure that the Al₂O₃ grain containing the finetetragonal ZrO₂ grain therein is trapped within the tetragonal ZrO₂grain, a residual stress field is locally generated around each of thefine tetragonal ZrO₂ grains within the Al₂O₃ grains and around each ofthe Al₂O₃ grains within the tetragonal ZrO₂ grains by a difference inthermal expansion coefficient between Al₂O₃ and ZrO₂ during a coolingprocedure after sintering. By the influence of the residual stressfield, lots of dislocations easily occur within the respective crystalgrains. The dislocations are then piled up with each other, and finallythe sub-grain boundaries are formed within the Al₂O₃ and ZrO₂ grains,respectively. The sub-grain boundaries provide the finer-grainedstructure, which has the capability of increasing a critical stressrequired for causing the stress-induced phase transformation from thetetragonal ZrO₂ to the monoclinic ZrO₂. As a result, the compositeceramic material of the present invention demonstrates excellent wearresistance and hardness as well as high mechanical strength andtoughness.

Referring to the SEM photograph of the FIGURE, the structure of thecomposite ceramic material of the present invention is more concretelyexplained. This SEM photograph shows that the above-described compositegrain exists in a uniformly mixed structure of normal tetragonal ZrO₂grains not having Al₂O₃ grains therein, and normal α-Al₂O₃ grains nothaving ZrO₂ grains therein. In addition, it shows that an Al₂O₃ graincontaining a fine ZrO₂ grain therein and Al₂O₃ grains not containing thefine ZrO₂ grain therein are dispersed within the large ZrO₂ grainconstructing this composite grain. Moreover, it shows that an Al₂O₃grain containing a fine ZrO₂ grain therein other than the compositegrain exists in the composite ceramic material. The number of fine ZrO₂grains in the single Al₂O₃ grain and the number of Al₂O₃ grains in thesingle ZrO₂ grain are not restricted. For example, a plurality of fineZrO₂ grains may be trapped in the single Al₂O₃ grain, or a plurality ofAl₂O₃ grains may be trapped in the single ZrO₂ grain.

As a preferred embodiment of the present invention, the zirconia phasemay contain another stabilizer such as MgO, CaO, TiO₂ and/or Y₂O₃ inaddition to CeO₂. For example, it is preferred to use 0.01 to 1 mol % ofTiO₂ and/or 0.01 to 0.5 mol % of CaO with respect to the total amount ofthe zirconia phase in addition to 10 to 12 mol % of CeO₂. In this case,the grain growth of the zirconia phase is enhanced to a moderate degreeby the addition of TiO₂, so that Al₂O₃ grains can be easily dispersedwithin the ZrO₂ grains. In addition, it is possible to increase acritical stress of the stress induced phase transformation. When theadditive amount of TiO₂ is less than 0.01 mol %, the effect of enhancingthe grain growth of the zirconia phase may be not obtained sufficiently.On the other hand, when the additive amount of TiO₂ exceeds 1 mol %,abnormal grain growth of the zirconia phase easily occurs, so that themechanical strength and the wear resistance of the composite ceramicmaterial may deteriorate.

On the other hand, the addition of CaO prevents the abnormal graingrowth of the zirconia phase to improve the balance between themechanical strength and toughness. In particular, it is effective toobtain the composite ceramic material having excellent wear resistanceand mechanical strength. When the additive amount of CaO is less than0.01 mol %, the effect of preventing the abnormal grain growth of thezirconia phase may be not obtained sufficiently. On the other hand, whenthe additive amount of CaO exceeds 0.5 mol %, cubic zirconia beings toappear in the zirconia phase, so that it becomes difficult to obtain thezirconia phase composed of 90 vol % or more of tetragonal ZrO₂. Thegeneration of cubic zirconia easily leads to an increase in averagegrain size of the zirconia phase. In such a case, deterioration inmechanical strength, toughness and wear resistance may occur. Thezirconia phase may contain a small amount of impurities. For example, itis desired that the amount of the impurities is 0.5 mol % or less withrespect to the total amount of the zirconia phase.

The composite ceramic material of the present invention is preferablyused in applications requiring excellent wear resistance expected byincreasing the Al₂O₃ content, while maintaining mechanical strength andtoughness substantially equal to the previous ZrO₂—Al₂O₃ ceramicmaterials. For example, it is preferred to use the composite ceramicmaterial of the present invention for an artificial joint described inthe international patent application WO02/11780. That is, when a jointportion of the artificial joint is provided by a sliding contact betweenthe composite ceramic material and polyethylene, it is possible toreduce a wear amount of polyethylene. In addition, when the jointportion of the artificial joint is formed by a sliding contact betweenthe composite ceramic materials, particularly improved wear resistancecan be achieved. Thus, by use of the composite ceramic material of thepresent invention, it is possible to obtain the artificial joint havingthe capability of stably providing a smooth joint motion for an extendedtime period under severe conditions in a living body.

Next, a method of producing the ZrO₂—Al₂O₃ composite ceramic material ofthe present invention is explained. The production method comprises thesteps of mixing a first power for providing the ZrO₂ phase with a secondpowder for providing the Al₂O₃ phase such that an amount of the Al₂O₃phase in the composite ceramic material is in a range of 20 to 70 vol %,molding a resultant mixture in a desired shape to obtain a greencompact, and sintering the green compact at a sintering temperature inan oxygen-containing atmosphere. The production method of the presentinvention is characterized in that the second powder contains thecomposite powder described above, which is composed of the Al₂O₃particles each having the fine ZrO₂ particle therein, and a ratio of thenumber of the Al₂O₃ particles each having the fine ZrO₂ particle thereinin the second powder is 5% or more.

To obtain the ZrO₂ phase composed of 90 vol % or more of tetragonalZrO₂, the first powder is prepared such that the ZrO₂ phase contains 10to 12 mol % of CeO₂ as a stabilizer. In addition, as the first powder,it is preferred to use a tetragonal ZrO₂ powder containing a requiredamount of TiO₂ and/or CaO in addition to CeO₂. A preparation process ofthe first powder is not restricted. For example, the following processis recommended.

That is, a cerium containing compound such as cerium salts is added toan aqueous solution of a zirconium salt. If necessary, an aqueoussolution of a titanium salt and/or a calcium salt, or an organicsolution of a titanium or calcium alkoxide as a titanium containingcompound or a calcium containing compound may be added. Then, hydrolysisis performed by adding an alkali aqueous solution such as aqueousammonia to a resultant mixture to obtain a precipitate. The precipitateis dried, calcined in the oxygen-containing atmosphere, e.g., in theair, and then pulverized by means of wet ball milling to obtain thetetragonal ZrO₂ powder having a desired particle distribution.

In the case of using the tetragonal ZrO₂ powder, it is preferred thatthe ZrO₂ powder has a specific surface area of 10 to 20 m²/g to obtainthe green compact of a sufficient green density. Such a green compactcan be easily sintered by pressureless sintering. When the specificsurface area is less than 10 m²/g, it becomes difficult to obtain theZrO₂ phase having an average grain size of 1 μm or less after sintering.On the other hand, when the specific surface area exceeds 20 m²/g, thebulk density considerably decreases, so that handling of the firstpowder becomes difficult. As a result, there is a fear that the greencompact cannot be densely sintered by pressureless sintering.

In the present invention, to uniformly disperse the composite grains inthe composite ceramic material, the second powder contains at least 5%of the composite powder composed of the Al₂O₃ particles each having thefine ZrO₂ particle therein. For example, a required amount of the firstpowder is mixed with an Al₂O₃ powder to obtain a mixed powder, and thena resultant mixed powder is calcined in an oxygen containing atmosphereat a temperature of 800° C. to 1300° C., and preferably 1000° C. to1200° C. to obtain the second powder containing the composite powder. Inthis case, it is preferred that the Al₂O₃ powder is at least oneselected from a θ-Al₂O₃ powder and a γ-Al₂O₃ powder having a specificsurface area of 50 to 400 m²/g. The specific surface area of this Al₂O₃powder is much greater than the specific surface of the first powder. Inother words, since the Al₂O₃ powder used to prepare the composite powderis much finer than the first powder, the above-described mixed powdercomprises ZrO₂ particles surrounded with ultra-fine Al₂O₃ particles.

Next, a phase transformation of θ-Al₂O₃ and/or γ-Al₂O₃ of the mixedpowder to α-Al₂O₃ occurs during the calcining procedure. At this time,the ZrO₂ particles in the mixed powder are trapped within α-Al₂O₃particles each having an increased particle size caused by the phasetransformation. The thus obtained composite powder is excellent inmoldability as compared with the case of using the θ-Al₂O₃ or γ-Al₂O₃powder. In addition, there is an advantage that the average grain sizeof the Al₂O₃ particle dispersed in the composite ceramic material can beeasily controlled in the range of 0.1 to 0.5 μm.

It is preferred that the composite powder obtained by the abovepreparation process is mainly composed of α-Al₂O₃ particles having anaverage grain size of 0.3 μm or less, each of which has the fine ZrO₂particle therein. However, an amount of α-Al₂O₃ in the composite powderis not restricted. That is, it is enough that a part of θ-Al₂O₃ and/orγ-Al₂O₃ is transformed to α-Al₂O₃ by the calcining procedure, andallowed to be mixed condition of θ-Al₂O₃ and/or γ-Al₂O₃ and α-Al₂O₃. Inthe present invention, the ratio of the number of the Al₂O₃ particles ofthe each having the fine ZrO₂ particle therein in the second powder is5% or more, preferably 25% or more, and more preferably 35% or more.

As another preferred process of preparing the second powder, a ZrO₂powder is added to an aqueous solution of an aluminum salt or an organicsolution of an aluminum alkoxide. A resultant mixture is hydrolyzed toobtain a precipitate, and then the precipitate is dried. The driedprecipitate is calcined in an oxygen containing atmosphere at atemperature of from 800° C. to 1300° C., and then pulverized by means ofwet ball milling to obtain the second powder having a desired particledistribution. In the above method, an aqueous solution of a zirconiumsalt may be used in stead of the ZrO₂ powder.

In the case of preparing the composite powder as the second powder, amixing ratio of Al₂O₃ and ZrO₂ is not restricted. To efficiently obtainthe α-Al₂O₃ particles each containing a fine ZrO₂ particle therein, itis preferred that a volume ratio of Al₂O₃:ZrO₂ in the composite powderis in a range of 95:5 to 50:50. When the value of ZrO₂ in this volumeratio is less than 5, it is difficult to obtain sufficient amounts ofthe α-Al₂O₃ particles each containing a fine ZrO₂ particle therein bythe calcining procedure. Consequently, the formation amount of thecomposite grains in the composite ceramic material decreases. On theother hand, when the value of ZrO₂ in this volume ratio is more than 50,an agglomeration of the ZrO₂ particles may occur. When the above volumeratio is within the range of 90:10 to 60:40, it is possible to moreefficiently obtain the α-Al₂O₃ particles each containing a fine ZrO₂particle therein, thereby providing a high-quality composite powdersuitable for producing the composite ceramic material of the presentinvention.

If necessary, a HIP treatment may be performed in an oxygen-containingatmosphere after sintering. To obtain effects of the HIP treatment atthe maximum, it is preferred that the sintered body of the compositeceramic material obtained by the pressureless sintering has a relativedensity of 95% or more. A concentration of oxygen in theoxygen-containing atmosphere is not restricted. A mixture gas of oxygenand an inert gas such as argon may be used. In this case, it ispreferred that the concentration of oxygen is approximately 5 vol % ormore with respect to a total volume of the mixture gas.

EXAMPLES

The present invention is concretely explained below according topreferred examples. These Examples are illustrative only, and thereforethe present invention is not limited to them.

Examples 1 to 6 and Comparative Examples 1 to 3

A ZrO₂—Al₂O₃ composite ceramic material of each of Examples 1 to 6 andComparative Examples 1 to 3 was produced by the following method. Thatis, as a first powder for providing a ZrO₂ phase of the compositeceramic material, a tetragonal ZrO₂ powder having a specific surfacearea of 15 m²/g and containing 11 mol % of CeO₂, 0.05 mol % of TiO₂, and0.16 mol % of CaO was used. On the other hand, a second powder forproviding an Al₂O₃ phase of the composite ceramic material was preparedby using a γ-Al₂O₃ powder having a specific surface area of 300 m²/g anda part of the tetragonal ZrO₂ powder. A mixture ratio by volume of theγ-Al₂O₃ powder and the tetragonal ZrO₂ powder is 70:30.

That is, required amounts of the γ-Al₂O₃ powder and the tetragonal ZrO₂powder were ball-milled in an ethanol solvent for 24 hours, and thendried to obtain a mixed powder. Subsequently, the mixed powder wascalcined at 1000° C. in the air for 2 hours. The thus obtained calcinedpowder was further ball-milled in an ethanol solvent for 24 hours, andthen dried to obtain an Al₂O₃ powder containing a composite powder(i.e., Al₂O₃ particles each having a ZrO₂ particle therein) as thesecond powder for providing the Al₂O₃ phase. From SEM observation, aratio of the composite powder in the thus obtained Al₂O₃ powder, i.e., aratio of the number of the Al₂O₃ particles each having a ZrO₂ particletherein relative to the entire Al₂O₃ particles of the thus obtainedAl₂O₃ powder is about 28.3%.

The remaining tetragonal ZrO₂ powder was mixed with the Al₂O₃ powdercontaining the composite powder such that an Al₂O₃ amount in thecomposite ceramic material is in a range of 10 to 80 vol %, as listed inTable 1. A resultant mixture was ball-milled in an ethanol solvent for24 hours, and then dried to obtain a powder for sintering. InComparative Example 1, the Al₂O₃ content is zero.

TABLE 1 ZrO₂ Average Grain phase (mol %) Al₂O₃ phase Size (μm) CeO₂ TiO₂CaO (vol %) ZrO₂ Al₂O₃ Comparative 11 0.05 0.16 0 2.50 — Example 1Comparative 11 0.05 0.16 10 1.35 0.23 Example 2 Example 1 11 0.05 0.1620 0.43 0.24 Example 2 11 0.05 0.16 30 0.23 0.26 Example 3 11 0.05 0.1640 0.21 0.27 Example 4 11 0.05 0.16 50 0.19 0.27 Example 5 11 0.05 0.1660 0.18 0.28 Example 6 11 0.05 0.16 70 0.17 0.29 Comparative 11 0.050.16 80 0.16 0.30 Example 3

The thus obtained powder for sintering was molded at the pressure of 10MPa by uniaxial pressing to obtain a disk-shaped green compact having adiameter of about 68 mm. After a CIP (cold isostatic pressing) treatmentwas performed to the green compact at the pressure of 147 MPa, the greencompact was sintered at the sintering temperature of 1440° C. for 3hours in the air by pressureless sintering to obtain a sintering body.

With respect to each of Examples 1 to 6 and Comparative Examples 1 to 3,the sintered body has a relative density of more than 99%. By X-raydiffraction analysis, it was confirmed that the ZrO₂ phase of therespective sintered body is composed of 90 vol % or more of tetragonalZrO₂ and the balance of monoclinic ZrO₂. In addition, from SEM (scanningelectron microscope) and TEM (transmission electron microscope)observations, it was confirmed that Al₂O₃ grains each having a fine ZrO₂grain therein are dispersed in the sintered body of each of Examples 1to 6 and Comparative Examples 2 and 3, and some of the Al₂O₃ grains eachhaving the fine ZrO₂ grain therein are trapped within ZrO₂ grains toform composite grains.

In addition, a first dispersion ratio (W1), which is defined as a ratioof the number of the Al₂O₃ grains each having the fine ZrO₂ graintherein relative to the number of the entire Al₂O₃ grains dispersed inthe composite ceramic material, a second dispersion ratio (W2), which isdefined as a ratio of the number of Al₂O₃ grains of the composite grainsrelative to the number of the entire Al₂O₃ grains dispersed in thecomposite ceramic material, and a third dispersion ratio (W3), which isdefined as a ratio of the number of the Al₂O₃ grains trapped within theZrO₂ grains relative to the number of the entire Al₂O₃ grains dispersedin the composite ceramic material, are listed in Table 2.

The first to third dispersion ratios (W1, W2, W3) were determined by thefollowing method. First, a sample for observation was prepared bypolishing the sintered body and performing a heat treatment to thepolished surface. Then, the SEM observation of the sample or the TEMobservation of the sintered body was performed to count the number (S1)of entire Al₂O₃ grains existing within a view field (e.g., it ispreferred to select an appropriate observation magnification such thatat least 1000 grains exist in the view field), the number (n1) of Al₂O₃grains each having the fine ZrO₂ grain therein in the same view field,the number (n2) of Al₂O₃ grains of the composite grains in the same viewfield, and the number (n3) of Al₂O₃ grains trapped within the ZrO₂grains in the same view field. By substituting these numbers to thefollowing equations, the first to third dispersion ratios werecalculated. Results are shown in Tables 2.

W1[%]=(n1/S1)×100

W2[%]=(n2/S1)×100

W3[%]=(n3/S1)×100

TABLE 2 Bending Fracture Vickers First Second Third Strength ToughnessHardness Wear factor Dispersion Dispersion Dispersion (MPa) (MPa ·m^(1/2)) (GPa) (mm³/Nm × 10⁻⁷) Ratio W1 (%) Ratio W2 (%) Ratio W3 (%)Comparative 750 18.0 8.5 230 — — — Example 1 Comparative 1080 17.2 9.748.2 40.0 1.4 3.5 Example 2 Example 1 1260 16.7 10.8 0.135 41.2 1.4 3.4Example 2 1380 15.8 12.0 0.048 39.4 1.3 3.3 Example 3 1430 14.5 13.10.036 38.7 1.2 3.1 Example 4 1410 13.2 14.3 0.028 39.3 1.1 2.8 Example 51340 11.8 15.4 0.051 39.1 0.9 2.3 Example 6 1220 10.2 16.6 0.074 41.20.7 1.7 Comparative 900 7.9 17.7 2.65 44.4 0.4 0.9 Example 3

In addition, with respect to each of Examples 1 to 6 and ComparativeExamples 1 to 3, average grain sizes of ZrO₂ grains and Al₂O₃ grains ofthe sintered body were measured by the TEM/SEM observation. To evaluatemechanical properties of the composite ceramic material, test specimenshaving the dimensions of 4 mm×3 mm×40 mm were prepared from the sinteredbody, and then 3-point bending strength and fracture toughness weremeasured at room temperature. The fracture toughness was measured by theIF method. Results are listed in Tables 1 and 2.

Moreover, to evaluate wear resistance of the composite ceramic material,a pin-on-disc test was performed in the presence of distilled water as alubricant. The pin and the disc are made of the same composite ceramicmaterial. The pin is provided with a cylinder solid having a diameter of5 min and a length of 15 mm, and a circular cone having an apical angleof 30° and formed on a top of the cylinder solid. The top end of thecircular cone has a flat mirror area with a diameter of 1.5 mm, which isused as a sliding surface. A surface roughness of this sliding surfaceis smaller than 0.005 μm Ra.

On the other hand, the disc has a diameter of 50 mm and a thickness of 8mm. A sliding surface of the disc to be made contact with the pin is amirror polished surface having a surface roughness smaller than 0.005 μmRa. After the pin was placed on a circumference having a radius of 22 mmfrom the disc center on the disc, the pin-on-disc test was performed ata disc rotational speed of 60 mm/sec. A load applied to the pin is 60N,and a sliding distance is constant (25 km). Since the diameter of thetop end of the pin is 1.5 mm, an initial friction pressure on the topend of the pin is 33 MPa. The pin-on-disc test was repeated three timesunder the same condition. An average value of the tests was adopted asdata.

A reduction in weight of the pin was measured after the test, and a wearfactor (Wf) was calculated by the following equation.

Wf=(W1−W2)/P·L·ρ

Where,

Wf: wear factor (mm3/Nm)W1: dry weight (g) of pin before testW2: dry weight (g) of pin after testP: load (N)L: sliding distance (m)ρ: density (g/mm3) of test specimen

In addition, Vickers hardness of the composite ceramic material wasmeasured. Measurement results of the wear factor and the hardness areshown in Table 2.

As understood from results of Tables 1 and 2, the sintered bodies ofExamples 1 to 6 containing 20 to 70 vol % of Al₂O₃ have the firstdispersion ratio (W1) greater than 39%, the second dispersion ration(W2) greater than 0.5%, and the third dispersion ratio (W3) greater than1.5%. In addition, these sintered bodies demonstrate excellentmechanical properties of a bending strength greater than 1200 MPa and afracture toughness higher than 10.0 MPa·m^(1/2).

On the other hand, since the sintered body of Comparative Example 1 doesnot contain the Al₂O₃ phase, it has excellent fracture toughness.However, the bending strength of the sintered body is considerably low.In the sintered body of Comparative Example 2, excellent fracturetoughness is obtained, and the first to third dispersion ratioscomparable to them of the sintered body of Example 1. However, theaverage grain size (=1.35 μm) of the ZrO₂ grains of Comparative Example2 is much greater than the average grain size (=0.43 μm) of the ZrO₂grains of Example 1. This suggests that the grain growth of the ZrO₂grains can not be sufficiently inhibited by using such a small amount ofAl₂O₃. As a result, the sintered body of Comparative Example 2 has arelatively low mechanical strength, and a tendency of increasingvariations in mechanical properties. Thus, it is difficult to providethe composite ceramic material that is excellent in both of strength andtoughness. With respect to the sintered body of Comparative Example 3,since the excessive amount of Al₂O₃ was used, both of strength andtoughness considerably lowered. In addition, the number of the Al₂O₃grains trapped within the ZrO₂ grains, i.e., the third dispersion ratio(W3) is extremely low. This suggests that the composite grains can notbe efficiently dispersed in the composite ceramic material in the caseof using such a large amount of Al₂O₃.

As described above, the concern of the present invention is to providethe ceramic material having excellent wear resistance and hardness,while maintaining high strength and toughness under the larger amount ofAl₂O₃ than heretofore. The results of Table 2 show that both of wearresistance and hardness can be highly improved when the Al₂O₃ content isin the range of 20 to 70 vol %. On the contrary, the sintered body ofComparative Example 2 shows a deterioration in wear resistance due tothe deficiency of Al₂O₃ and the increase in average grain size of theZrO₂ grains. In addition, the sintered body of Comparative Example 3shows poor mechanical strength and toughness as well as thedeterioration in wear resistance due to the excessive amount of Al₂O₃ inthe composite ceramic material.

Examples 7 to 21

A ZrO₂—Al₂O₃ composite ceramic materials of each of Examples 7 to 21 wasproduced by the following method. That is, as listed in table 3, atetragonal ZrO₂ powder having a specific surface area of 15 m²/g andcontaining a CeO₂ amount of 10 to 12 mol % or containing the CeO₂ amountof 10 to 12 mol %, TiO₂ amount of 0.02 to 1 mol %, and a CaO amount of0.02 to 0.5 mol % was used as a first powder for providing a ZrO₂ phaseof the composite ceramic material.

TABLE 3 ZrO₂ Average Grain phase (mol %) Al₂O₃ phase Size (μm) CeO₂ TiO₂CaO (vol %) ZrO₂ Al₂O₃ Example 7 10 0.00 0.00 40 0.21 0.20 Example 8 100.05 0.03 40 0.24 0.22 Example 9 10 0.30 0.15 40 0.33 0.25 Example 10 100.70 0.35 40 0.45 0.28 Example 11 10 1.00 0.50 40 0.50 0.30 Example 1211 0 0 40 0.21 0.20 Example 13 11 0.03 0.02 40 0.23 0.21 Example 14 110.20 0.10 40 0.29 0.24 Example 15 11 0.60 0.30 40 0.43 0.28 Example 1611 0.90 0.45 40 0.48 0.29 Example 17 12 0 0 40 0.21 0.20 Example 18 120.02 0.01 40 0.22 0.21 Example 19 12 0.10 0.05 40 0.26 0.23 Example 2012 0.50 0.25 40 0.40 0.27 Example 21 12 0.80 0.40 40 0.47 0.29

On the other hand, a second powder for providing an Al₂O₃ phase of thecomposite ceramic material was prepared by the following process. Thatis, a part of the above-described tetragonal ZrO₂ powder was added to ahydrochloric acid solution of aluminum chloride (AlCl₃) such that amixture ratio by volume of Al₂O₃:ZrO₂ is 70:30. Next, an aqueoussolution of sodium hydroxide was added to a resultant mixed solution,and hydrolyzed to obtain a precipitate. The precipitate was washed withwater, and then dried. Next, the dried precipitate was calcined at 1000°C. in the air for 2 hours. The thus obtained calcined powder wasball-milled in an ethanol solvent for 24 hours, and then dried to obtainan Al₂O₃ powder containing a composite powder. From SEM observation, aratio of the composite powder in the thus obtained Al₂O₃ powder, i.e., aratio of the number of the Al₂O₃ particles each having a ZrO₂ particletherein relative to the entire Al₂O₃ particles of the thus obtainedAl₂O₃ powder is about 39.8%.

The remaining tetragonal ZrO₂ powder was mixed with the Al₂O₃ powdercontaining the composite powder such that an Al₂O₃ amount in thecomposite ceramic material is 40 vol %. A resultant mixture wasball-milled in an ethanol solvent for 24 hours, and then dried to obtaina powder for sintering. The powder for sintering was molded into arequired shape by uniaxial pressing, and then sintered by pressurelesssintering to obtain a sintered body, as in the case of Example 1.

With respect to each of Examples 7 to 21, the sintered body has arelative density of more than 99%. By X-ray diffraction analysis, it wasconfirmed that the ZrO₂ phase of the respective sintered body iscomposed of 90 vol % or more of tetragonal ZrO₂ and the balance ofmonoclinic ZrO₂. In addition, from SEM and TEM observations, it wasconfirmed that Al₂O₃ grains each having a fine ZrO₂ grain therein aredispersed in the sintered body of each of Examples 7 to 21, and some ofthe Al₂O₃ grains each having the fine ZrO₂ grain therein are trappedwithin ZrO₂ grains to form composite grains. In addition, according tothe same method as Example 1, the first to third dispersion ratios weredetermined with respect to each of Examples 7 to 21. Results are shownin Table 4.

TABLE 4 Bending Fracture First Second Third Strength ToughnessDispersion Dispersion Dispersion (MPa) (MPa · m^(1/2)) Ratio W1 (%)Ratio W2 (%) Ratio W3 (%) Example 7 1100 18.0 56.5 1.3 2.3 Example 81350 17.6 57.1 1.6 2.8 Example 9 1260 17.4 56.7 1.7 3.0 Example 10 124017.2 58.1 1.8 3.1 Example 11 1210 17.1 57.6 1.9 3.3 Example 12 1220 14.856.5 1.3 2.3 Example 13 1420 14.5 57.1 1.6 2.8 Example 14 1410 14.3 55.21.6 2.9 Example 15 1310 14.1 56.7 1.7 3.0 Example 16 1290 14.0 56.3 1.83.2 Example 17 1410 11.0 56.5 1.3 2.3 Example 18 1570 10.7 57.1 1.6 2.8Example 19 1540 10.6 58.6 1.7 2.9 Example 20 1380 10.5 56.7 1.7 3.0Example 21 1360 10.4 56.3 1.8 3.2

In addition, with respect to each of Examples 7 to 21, average grainsizes of the ZrO₂ grains and the Al₂O₃ grains of the sintered body weremeasured by the SEM/TEM observation. The average grain size of the ZrO₂grains is in a range of 0.2 to 0.5 and the average grain size of theAl₂O₃ grains is 0.3 μm or less. To evaluate mechanical properties of thecomposite ceramic material, test specimens having dimensions of 4 mm×3mm×40 mm were prepared from the sintered body, and 3-point bendingstrength and fracture toughness were measured at room temperature. Thefracture toughness was measured by the IF method. Results are shown inTables 3 and 4.

The results of Tables 3 and 4 suggest that the bending strength can befurther improved by using slight amounts of TiO₂ and CaO in addition toCeO₂ as the stabilizer, without deteriorating the fracture toughness.

Examples 22 to 27

A ZrO₂—Al₂O₃ composite ceramic materials of each of Examples 22 to 27was produced by the following method. That is, as a first powder forproviding a ZrO₂ phase of the composite ceramic material, a tetragonalZrO₂ powder having a specific surface of 15 m²/g and containing 11 mol %of CeO₂, 0.05 mol % of TiO₂, and 0.13 mol % of CaO was used. On theother hand, a second powder for providing an Al₂O₃ phase of thecomposite ceramic material was prepared by using a θ-Al₂O₃ powder havinga specific surface of 100 m²/g and a part of the above-describedtetragonal ZrO₂ powder. A mixture ratio by volume of the θ-Al₂O₃ powderand the tetragonal ZrO₂ powder was changed in a range of 95:5 to 50:50,as shown in Table 5.

TABLE 5 ZrO₂ phase Al₂O₃ phase Average Grain (mol %) vol % CompositeSize (μm) CeO₂ TiO₂ CaO (Al₂O₃:ZrO₂) Powder Ratio (%) ZrO₂ Al₂O₃ Example22 11 0.05 0.13 50 (95:5)  7.2 0.25 0.30 Example 23 11 0.05 0.13 50(90:10) 14.0 0.23 0.28 Example 24 11 0.05 0.13 50 (80:20) 27.1 0.22 0.27Example 25 11 0.05 0.13 50 (70:30) 39.4 0.21 0.26 Example 26 11 0.050.13 50 (60:40) 52.5 0.22 0.27 Example 27 11 0.05 0.13 50 (50:50) 63.20.23 0.28

TABLE 6 Bending Fracture First Second Third Strength ToughnessDispersion Dispersion Dispersion (MPa) (MPa · m^(1/2)) Ratio W1 (%)Ratio W2 (%) Ratio W3 (%) Example 22 1230 13.0 10.3 0.3 2.9 Example 231340 13.1 20.0 0.6 3.0 Example 24 1430 13.3 38.7 1.2 3.1 Example 25 146013.3 56.3 1.8 3.2 Example 26 1480 13.2 75.0 2.4 3.2 Example 27 1500 13.190.3 2.8 3.1

That is, required amounts of the θ-Al₂O₃ powder and the above-describedtetragonal ZrO₂ powder were ball-milled in an ethanol solvent for 24hours, and then dried to obtain a mixed powder. Subsequently, the mixedpowder was calcined at 1000° C. in the air for 2 hours. The thusobtained calcined powder was further ball-milled in an ethanol solventfor 24 hours, and then dried to obtain an Al₂O₃ powder containing acomposite powder. From SEM observation, a ratio of the composite powderin the thus obtained Al₂O₃ powder, i.e., a ratio of the number of theAl₂O₃ particles each having a ZrO₂ particle therein relative to theentire Al₂O₃ particles of the thus obtained Al₂O₃ powder was estimated.Results are listed as “composite power ratio” in TABLE 5.

The remaining tetragonal ZrO₂ powder was mixed with the Al₂O₃ powdercontaining the composite powder such that an Al₂O₃ amount in thecomposite ceramic material is 50 vol %. A resultant mixture wasball-milled in an ethanol solvent for 24 hours, and the dried to obtaina powder for sintering. The powder for sintering was molded into arequired shape by uniaxial pressing, and then sintered by pressurelesssintering to obtain a sintered body, as in the case of Example 1.

With respect to each of Examples 22 to 27, the sintered body has arelative density of more than 99%. By X-ray diffraction analysis, it wasconfirmed that the ZrO₂ phase of the respective sintered body iscomposed of 90 vol % or more of tetragonal ZrO₂ and the balance ofmonoclinic ZrO₂. In addition, by SEM (scanning electron microscope) andTEM (transmission electron microscope) observations, it was confirmedthat Al₂O₃ grains each having a fine ZrO₂ grain therein are dispersed inthe sintered body of each of Examples 22 to 27, and some of the Al₂O₃grains each having the fine ZrO₂ grain therein are trapped within ZrO₂grains to form composite grains. In addition, according to the samemethod as Example 1, the first to third dispersion ratios weredetermined with respect to each of Examples 22 to 27. Results are shownin Table 6.

In addition, with respect to each of Examples 22 to 27, average grainsizes of ZrO₂ grains and Al₂O₃ grains of the sintered body were measuredby the SEM/TEM observation. The average grain size of the ZrO₂ grains isin a range of 0.2 to 0.3 μm, and the average grain size of the Al₂O₃grains is 0.3 μm or less. To evaluate mechanical properties of thecomposite ceramic material, test specimens having dimensions of 4 mm×3mm×40 mm were prepared from the sintered body, and 3-point bendingstrength and fracture toughness were measured at room temperature. Thefracture toughness was measured by the IF method. Results are shown inTables 5 and 6.

The results of Tables 5 and 6 suggest that when the mixture ratio ofAl₂O₃ and tetragonal ZrO₂ in the composite powder is in the range of95:5 to 50:50, and particularly 90:10 to 60:40, the ZrO₂ grains can beefficiently trapped within the Al₂O₃ grains. Furthermore, by selectingan appropriate mixture ratio of Al₂O₃ and tetragonal ZrO₂ in thecomposite powder, it is possible to obtain the composite ceramicmaterial having a further improved strength, while keeping the toughnessconstant.

Examples 28 to 32 and Comparative Examples 4 to 8

A ZrO₂—Al₂O₃ composite ceramic material of each of Examples 28 to 32 wasproduced by the following method. That is, as a first powder forproviding a ZrO₂ phase of the composite ceramic material, a tetragonalZrO₂ powder having a specific surface area of 15 m²/g and containing 11mol % of CeO₂ and 0.02 mol % of Y₂O₃ was used. On the other hand, asecond powder for providing an Al₂O₃ phase of the composite ceramicmaterial was prepared by using a spherical γ-Al₂O₃ powder having aspecific surface area of 100 m²/g and a part of the tetragonal ZrO₂powder. A mixture ratio by volume of the γ-Al₂O₃ powder and thetetragonal ZrO₂ powder is 50:50.

That is, required amounts of the spherical γ-Al₂O₃ powder and thetetragonal ZrO₂ powder were ball-milled in an ethanol solvent for 24hours, and then dried to obtain a mixed powder. Subsequently, the mixedpowder was calcined at 1000° C. in the air for 2 hours. The thusobtained calcined powder was further ball-milled in an ethanol solventfor 24 hours, and then dried to obtain an Al₂O₃ powder containing acomposite powder. From SEM observation, a ratio of the composite powderin the thus obtained Al₂O₃ powder, i.e., a ratio of the number of theAl₂O₃ particles each having a ZrO₂ particle therein relative to theentire Al₂O₃ particles of the thus obtained Al₂O₃ powder is about 62%.

The remaining tetragonal ZrO₂ powder was mixed with the Al₂O₃ powdercontaining the composite powder such that an Al₂O₃ amount in thecomposite ceramic material is in a range of 30 to 70 vol %, as listed inTable 7. A resultant mixture was ball-milled in an ethanol solvent for24 hours, and then dried to obtain a powder for sintering.

TABLE 7 Al₂O₃ ZrO₂ phase (mol %) phase Average Grain Size (wm) CeO₂ Y₂O₃(vol %) ZrO₂ Al₂O₃ Example 28 11 0.02 30 0.22 0.25 Example 29 11 0.02 400.20 0.26 Example 30 11 0.02 50 0.18 0.26 Example 31 11 0.02 60 0.170.27 Example 32 11 0.02 70 0.16 0.28 Comparative 11 0.02 80 0.15 0.29Example 4 Comparative 11 0.02 40 0.23 0.30 Example 5 Comparative 11 0.0250 0.21 0.30 Example 6 Comparative 11 0.02 60 0.20 0.33 Example 7Comparative 11 0.02 70 0.19 0.38 Example 8

The obtained powder for sintering was molded at the pressure of 10 MPaby uniaxial pressing to obtain a disk-shaped green compact having adiameter of about 68 mm. After a CIP (cold isostatic pressing) treatmentwas performed to the green compact at the pressure of 147 MPa, the greencompact was sintered at the sintering temperature of 1440° C. for 3hours in the air by pressureless sintering to obtain a sintering body.

On the other hand, a ZrO₂—Al₂O₃ composite ceramic material ofComparative Example 4 was produced by the same method as Examples 28except that the composite powder was added such that the content of theAl₂O₃ phase in the composite ceramic material is 80 vol %. In addition,ZrO₂—Al₂O₃ composite ceramic materials of Comparative Examples 5 to 8were produced by the same methods as Examples 29 to 32, respectively,except that a commercially available α-Al₂O₃ powder having an averageparticle size of 0.2 μm was used in place of the Al₂O₃ powder containingthe composite powder.

With respect to each of Examples 28 to 32 and Comparative Examples 4 to8, the sintered body has a relative density of more than 99%. By X-raydiffraction analysis, it was confirmed that the ZrO₂ phase of therespective sintered body is composed of 90 vol % or more of tetragonalZrO₂ and the balance of monoclinic ZrO₂. In addition, from SEM (scanningelectron microscope) and TEM (transmission electron microscope)observations, it was confirmed that Al₂O₃ grains each having a fine ZrO₂grain therein are dispersed in the sintered body of each of Examples 28to 32 and Comparative Example 4, and some of the Al₂O₃ grains eachhaving the fine ZrO₂ grain therein are trapped within ZrO₂ grains toform composite grains. In addition, according to the same method asExample 1, the first to third dispersion ratios were determined withrespect to each of Examples 28 to 32 and Comparative Examples 4 to 8.Results are shown in Table 8.

TABLE 8 Bending Fracture Vickers First Second Third Strength ToughnessHardness Dispersion Dispersion Dispersion (MPa) (MPa · m^(1/2)) (GPa)Ratio W1 (%) Ratio W2 (%) Ratio W3 (%) Example 28 1480 16.8 12.6 89.03.0 3.4 Example 29 1560 16.2 13.8 87.5 2.8 3.2 Example 30 1580 15.5 15.088.8 2.6 2.9 Example 31 1550 14.5 16.2 88.4 2.1 2.4 Example 32 1430 12.817.4 89.3 1.6 1.8 Comparative 1080 9.8 18.6 88.5 0.9 1.0 Example 4Comparative 1050 5.7 12.9 3.6 0.0 3.1 Example 5 Comparative 970 5.3 14.13.1 0.0 2.8 Example 6 Comparative 880 4.9 15.2 2.6 0.0 2.3 Example 7Comparative 780 4.5 16.3 2.0 0.0 1.7 Example 8

As understood from results of Tables 7 and 8, the sintered bodies ofExamples 28 to 32 containing 30 to 70 vol % of Al₂O₃ have the firstdispersion ratio (W1) greater than 88%, the second dispersion ration(W2) greater than 1.5%, and the third dispersion ratio (W3) of 1.8% ormore. In addition, these sintered bodies demonstrate excellentmechanical properties of a bending strength greater than 1400 MPa and afracture toughness higher than 12 MPa·m^(1/2).

On the other hand, in Comparative Example 4, the bending strength andthe fracture toughness of the sintered body deteriorated due to theexcessive amount (80 vol %) of the Al₂O₃ phase, despite an increase inthe first dispersion ratio. In Comparative Examples 5 to 8, since thesintered bodies were produced without using the composite power, thefirst and second dispersion ratios considerably lowered, anddeteriorations in bending strength and fracture toughness of thesintered body occurred. Thus, it becomes difficult to obtain thecomposite ceramic material having excellent bending strength andfracture toughness.

INDUSTRIAL APPLICABILITY

As understood from the above Examples, the ZrO₂—Al₂O₃ composite ceramicmaterial of the present invention is characterized in that Al₂O₃ grainseach having a fine ZrO₂ grain therein are dispersed in the compositeceramic material at a higher dispersion ratio than heretofore, and someof the Al₂O₃ grains each having the fine ZrO₂ grain therein are trappedwithin larger-sized ZrO₂ grains to form composite grains. The formationof such a nanocomposite structure provides further improvements in wearresistance, hardness, strength and toughness of the ZrO₂—Al₂O₃ ceramicmaterial under a larger amount of Al₂O₃ than heretofore.

Therefore, the composite ceramic material of the present invention isexpected to be preferably utilized in various application fields, forexample, parts for industrial machine such as ferrule for optical fiberconnector, bearings and dies, cutting tools such as scissor and sawblade, stationery goods, chemical goods such as mechanical seal andmilling media, goods for sport, medical equipments such as surgicalknife, biomaterial parts such as artificial joint, artificial bone,artificial dental root, abutment and crown.

1. A ZrO₂—Al₂O₃ composite ceramic material comprising a ZrO₂ phasecontaining 10 to 12 mol % of CeO₂ as a stabilizer and composed of 90 vol% or more of tetragonal ZrO₂, and an Al₂O₃ phase, wherein an amount ofsaid Al₂O₃ phase in the composite ceramic material is in a range of 30to 70 vol %, Al₂O₃ grains each having a fine ZrO₂ grain therein aredispersed in the composite ceramic material, some of the Al₂O₃ grainseach having the fine ZrO₂ grain therein are trapped within ZrO₂ grainsto form composite grains, a first dispersion ratio defined as a ratio ofthe number of the Al₂O₃ grains each having the fine ZrO₂ grain thereinrelative to the number of the entire Al₂O₃ grains dispersed in thecomposite ceramic material is 50% or more, and a second dispersion ratiodefined as a ratio of the number of the Al₂O₃ grains of said compositegrains relative to the number of the entire Al₂O₃ grains dispersed inthe composite ceramic material is 0.3% or more.
 2. The composite ceramicmaterial as set forth in claim 1, wherein ZrO₂ grains each having anAl₂O₃ grain therein are dispersed in the composite ceramic material, athird dispersion ratio defined as a ratio of the number of the Al₂O₃grains trapped within the ZrO₂ grains relative to the number of theentire Al₂O₃ grains dispersed in the composite ceramic material is 1.5%or more, and wherein the second dispersion ratio “R2” is 0.6% or more,and wherein the second dispersion ratio “R2” satisfies the followingcondition:R2≧α×R3×R1/100 wherein “R1” is the first dispersion ratio, “R3” is thethird dispersion ratio, and “a” is a coefficient determined within arange of 0.8 to 1 by taking account of measurement errors.
 3. (canceled)4. A method of producing the ZrO₂—Al₂O₃ composite ceramic material asset forth in claim 1 comprising the steps of: mixing a first powder forproviding said ZrO₂ phase with a second powder for providing said Al₂O₃phase such that an amount of said Al₂O₃ phase in the composite ceramicmaterial is in a range of 30 to 70 vol %; molding a resultant mixture ina desired shape to obtain a green compact; and sintering said greencompact in an oxygen-containing atmosphere, wherein the second powdercontains Al₂O₃ particles each having a fine ZrO₂ particle therein,wherein some of the Al₂O₃ particles each having the fine ZrO₂ particletherein becomes trapped within ZrO₂ particles to form composite grains,wherein a ratio of the number of the Al₂O₃ particles each having thefine ZrO₂ particle therein relative to the number of entire Al₂O₃particles dispersed in the composite ceramic material is 50% or more,and wherein a ratio of the number of the Al₂O₃ particles of saidcomposite grains relative to the number of the entire Al₂O₃ particlesdispersed in the composite ceramic material is 0.3% or more.
 5. Themethod as set forth in claim 4, wherein a preparation process of thesecond powder comprises the step of adding a ZrO₂ powder to at least oneselected from a θ-Al₂O₃ powder and a γ-Al₂O₃ powder having a specificsurface area of 50 to 400 m²/g to obtain a mixed powder.
 6. The methodas set forth in claim 4, wherein a preparation process of the secondpowder comprises the steps of adding a ZrO₂ powder to one of an aqueoussolution of an aluminum salt and an organic solution of an aluminumalkoxide, hydrolyzing a resultant mixture to obtain a precipitate, anddrying the precipitate.
 7. The method as set forth in claim 4, wherein apreparation process of the second powder comprises the steps of addingan aqueous solution of a zirconium salt to one of an aqueous solution ofan aluminum salt and an organic solution of an aluminum alkoxide,hydrolyzing a resultant mixture to obtain a precipitate, and drying theprecipitate.
 8. The method as set forth in claim 5, comprising the stepof calcining the mixed powder in an oxygen containing atmosphere at atemperature of 800° C. to 1300° C.
 9. The method as set forth in claim6, comprising the step of calcining the precipitate in an oxygencontaining atmosphere at a temperature of from 800° C. to 1300° C. 10.The method as set forth in claim 7, comprising the step of calcining theprecipitate in an oxygen containing atmosphere at a temperature of from800° C. to 1300° C.
 11. The method as set forth in claim 4, wherein avolume ratio of Al₂O₃:ZrO₂ in the second powder is in a range of 95:5 to50:50.
 12. The composite ceramic material as set forth in claim 1,wherein ZrO₂ grains each having an Al₂O₃ grain therein are dispersed inthe composite ceramic material, a third dispersion ratio defined as aratio of the number of the Al₂O₃ grains trapped within the ZrO₂ grainsrelative to the number of the entire Al₂O₃ grains dispersed in thecomposite ceramic material is 1.5% or more.